This invention relates to removal of liquid from suspensions and, more particularly, to a new and improved liquid removal system which provides more efficient and effective liquid removal.
In many conventional systems for separating suspended solids from a liquid, the treatment is carried out in three stages. In the first stage, the suspension is supplied to a settling device where gravity separation of suspended material from the liquid occurs over a period of time. Such gravity separation may be enhanced by flocculation or coagulation of the suspended material to collect fine particles into larger aggregates and by introduction of air or gas bubbles to enhance the gravity separation process. Such arrangements are described, for example, in U.S. Pat. Nos. 1,056,548, 1,376,459, 4,289,628 and 4,340,487.
In the second stage, the suspended material received from the gravity settler, which may have a total solids content of 1% or 2%, for example, is applied to the porous surface of a moving belt, screen or drum, through which liquid is drained to produce a layer of material having a substantially higher solids content, such as about 4% to 10%. In the third stage, the second stage output is squeezed in a filter press, belt filter press, or other dewatering device to remove most of the remaining liquid, producing a filter cake having, for example, about 20% to 50% solids content.
In apparatus for removing suspended material from a suspension having about 1% to 10% solids content, such as, for example, the apparatus used for the second stage of the above-described process, in which the suspension is applied to a porous surface to drain the liquid from the solid material, the porous surface often becomes clogged with the solid material from the suspension, thus inhibiting drainage and trapping liquid in and above the layer of solid material. As a result, the material removed from the porous surface and supplied to a filter press has a relatively high content of liquid which must be removed in the filter press. Although some gravity separators providing gaseous flotation, such as those described in Pat. Nos. 3,479,281 and 3,637,490, have included porous belts for conveying a low-solids-content scum or sludge from the upper level of a liquid-solids mixture in a gravity separator to a discharge point, such arrangements do not provide adequate drainage of the liquid carried by the sludge.